In tuning systems of the type used in television receivers utilizing voltage controlled reactive elements as frequency determining components, the frequency of operation is typically determined by a variable bias voltage provided by, for example, a potentiometer. Typically, a plurality of potentiometers are provided with each potentiometer being set to provide the proper bias voltage for a respective channel. The voltage controlled frequency determining element or elements are typically diodes with voltage controllable capacitances such as varactor diodes. For reasons of economy it is generally desired to utilize a single diode or circuit arrangement for all of the frequencies in which received signals are expected. In the case of a VHF tuner received signals can be expected in any of channels 2-13, however, band-switching circuitry can be used to divide the operation between low-band and high-band VHF channels. In the case of a UHF tuner received signals can be expected in any of channels 14-83, and accordingly, the frequency of operation of the tuner must be variable over the entire UHF band which includes video carrier frequencies from 471.25 MHz to 885.25 MHz.
Typical tuners using varactor-type diodes require a substantial range of capacitance change of the varactor diode to cover the frequency range of interest. The capacitance characteristic of typical varactor diodes, however, is not linear with respect to the applied bias voltage. While this problem is not unduly limiting with respect to the applied tuning bias voltage, the automatic frequency control (AFC) of the tuner is deleteriously affected. The AFC circuit provides a given correction voltage for a given frequency deviation from the proper intermediate frequency. Since the varactor diode capacitance characteristic is non-linear, however, a given AFC correction voltage provides a differing amount of correction depending upon the bias voltage or channel received thereby resulting in a varying AFC pull-in range. For example, if the proper pull-in range is provided at the low-frequency channels in the band of interest, the pull-in range at the high frequency channels becomes insufficient. Conversely, if the proper or desired pull-in range is provided for the high frequency channels, the pull-in range at the low frequency channels becomes excessive which can lead to such deleterious effect as locking onto the associated sound carrier, locking onto adjacent channel carriers and similar problems.
Furthermore, it has been found that when the tuning voltage is derived from the tap of a potentiometer connected in parallel with a zener diode in accordance with a typical prior art technique, the output resistance of the tuning voltage source varies greatly over the frequency range of interest. It has been further found that in accordance with this prior art technique the variation in output resistance varies the AFC correction voltage in such a manner that the maximum correction is provided at approximately the midrange of the potentiometer while minimum correction is provided at the extremities of the tuning range. This means that at high frequency channels where the tuning sensitivity is lowest, the low output impedance of the tuning voltage source tends to shunt the AFC source to decrease the correction and the pull-in range for the high frequency channels.
Prior art attempts to solve the above-noted and other problems have resulted in added circuitry with attendant added complexity, lower reliability, and higher cost, as well as performance deficiencies. Furthermore, prior art techniques have generally been unsuccessful in solving the problem of providing substantially constant or compensated pull-in performance of the AFC system.